Seismic events are common place occurrences in most parts of the world and happen on a wide range of scales from the smallest vibrations to the largest earthquakes. While measuring large seismic events is a fairly straightforward exercise, accurately measuring smaller seismic events is more challenging.
Oil-field drilling, stimulation, and production activities often cause microearthquakes (microseismic events), either by compacting rock, propagating fractures, or relieving shear stress. Thus, induced seismicity monitoring is one of the primary methods that field operators use to visualize reservoir conditions during enhanced recovery operations. For example, frequency, intensity, and spatial distribution of microseismic events may reveal valuable information about the chemical, hydraulic, and/or mechanical processes occurring in the volume around boreholes in the earth. Because high acoustic frequencies attenuate as they propagate through the rock, only borehole-placed sensors can provide necessary spatial resolution. However, many of the fields that are candidates for enhanced recovery operations also exhibit harsh environmental conditions in which existing seismic sensors are known to fail. Temperatures can be well outside the engineering specifications of traditional sensors, which rely on electronics to detect and communicate earth motion. Corrosive fluids generated by enhanced recovery methods further threaten sensors, especially at high temperatures where connectors and seals are less reliable.
Optical interferometry is a common technique used to measure small displacements of an object relative to a reference and are used in a wide variety of instruments, including gyroscopes, accelerometers and quantum cryptography systems to name a few. Optical interferometers based on optical fibers in particular have proven useful in many systems and instruments because the optical fibers can be easily manipulated and controlled.
Optical fiber technologies have recently become available for use in conventional oil and gas research, where they have shown great reliability in distributed temperature sensing and long range signal transmission. However, conventional optical seismic sensors are suitable for lower temperature environments such as towed arrays of hydrophones, ocean bottom arrays, or moderate temperature boreholes.
Thus, while optical interferometry has been shown promise for detecting seismic events, conventional approaches still present a number of challenges. For example, the temperature in a deep borehole formed the earth can be 300° C. or greater, far exceeding temperature ranges for reliable operation of electronic equipment. In addition, there is a need for sensor minimizing crosstalk between various directions of seismic disturbance to achieve acceptable levels of accuracy of measuring seismic data.